Compared with polycarbonates, which are currently used for the production of optical data storage media, polymers based on vinylcyclohexane which exhibit satisfactory mechanical properties have a higher viscosity at the same temperature over a broad range of low shear rates.
The accurate moulding of pits which are smaller and disposed more closely together than those cited in EP-A 317 263 and U.S. Pat. No. 4,911,966, and of the grooves which are possible nowadays, is essential for high densities of data storage of  greater than 5 Gbytes, particularly  greater than 10 Gbytes.
The method of producing polymers based on vinylcyclohexane which is described in EP-A 317 263 and the use thereof as substrates for optical discs result in a molecular weight which is too low compared with that which would ensure the operationally reliable production thereof (comparative example 1). The mechanical properties of the homopolymers described there are not very suitable for the production of optical data storage media.
The method described in U.S. Pat. No. 4,911,966 only results in partially hydrogenated products ( less than 97%), and most of the examples comprise degrees of hydrogenation of  less than 86%. According to the prior art, partially hydrogenated systems exhibit inadequate transparency (DE-AS 11 31 885 (=GB 933 596)). The partially hydrogenated systems which are disclosed are turbid, and are unsuitable for applications as optical substrates which are penetrated by a laser beam. Partially hydrogenated systems also have the disadvantage that their glass transition temperature depends on the degree of hydrogenation. In an industrial process, an adjustment of the degree of hydrogenation and thus of the thermal properties of the optical substrate can only be reproducibly implemented by expending considerable engineering effort and at considerable cost.
Moreover, for the most part the partially hydrogenated products which are cited in U.S. Pat. No. 4,911,966 have a molecular weight which is far too low for the operationally reliable production of substrates for optical data storage media.
The aforementioned patent specifications do not mention the quality of moulding of pit and groove structures or their existence in principle by means of the substrates cited there.
Optimised molecular weights and molecular weight distributions are essential for satisfactory mechanical properties, and at the same time good melt flow properties are essential for the moulding of pit and groove structures of high-density optical data storage media.
A molecular weight which is possibly too high can lead to problems with the moulding of pits and grooves, as a consequence of too high a viscosity.
The substrates according to the present invention, which comprise a polymer based on vinylcyclohexane with a narrow molecular weight distribution or a mixture thereof with a low molecular weight component, are distinguished by their good level of mechanical properties and by their good melt flow properties.
It is thereby possible to produce optical discs in an operationally reliable manner by injection moulding, and the discs can subsequently be handled without their bending or breaking.
Thinner substrates can be produced which have layer thicknesses less than 1.1 mm, of 0.6 mm thickness for example, and which at the same time exhibit satisfactory mechanical properties.
On account of these properties, the materials can be used very satisfactorily as substrates for optical data storage media.
Satisfactory mechanical properties are also required for other optical substrates which do not require mechanical indentations in the form of pits and grooves, for example, and have to be accompanied by a low level of birefringence, a low moment of inertia, a high level of dimensional stability when hot, a high modulus of elasticity, low water absorption and low density. These requirements are also met by the substrates according to the invention.
The present invention relates to polymers of vinylcyclohexane with an absolute molecular weight Mw from 100,000 to 450,000 g/mol or a mixture thereof with a low molecular weight component with an absolute molecular weight from 1000 to less than 100,000 g/mol, wherein the molecular weight distribution is characterised by a polydispersity index (PDI=Mw/Mn) of 1 to 3 and the maximum melt viscosity is 1000 Paxc2x7s, as measured at 300xc2x0 C. and at a shear rate of 1000 secxe2x88x921.
Any oligomeric fraction with a molecular weight Mw of up to 3000 g/mol which may possibly be present is not taken into account in the calculation of the polydispersivity index.
Any oligomeric fraction with a molecular weight of up to 3000 g/mol which may be present amounts to less than 5% of the weight of the polymer.
The molecular weight Mw of the high molecular weight polymer (homopolymer) preferably ranges from 200,000 to 450,000 gmolxe2x88x921, and ranges in particular from 200,000 to 400,000 gmolxe2x88x921.
The molecular weight Mw of a high molecular weight copolymer or block polymer preferably ranges from 100,000 to 400,000 gmolxe2x88x921, and ranges in particular from 100,000 to 250,000 gmolxe2x88x921.
The molecular weight Mw of the low molecular weight component generally ranges from 1000 to 100,000 gmolxe2x88x921, preferably from 7000 to 90,000 gmolxe2x88x921, most preferably from 10,000 to 90,000 gmolxe2x88x921.
The molecular weight distribution of the respective component is characterised by a polydispersity index (PDI=Mw/Mn) from 1 to 3.
In the case of mixtures, the proportion of low molecular weight component with respect to the weight of the mixture of high molecular weight and low molecular weight polymers generally amounts to up to 70% by weight, preferably 5 to 60% by weight, most preferably 10 to 50% by weight.
A polymer based on vinylcyclohexane is preferred both for the high molecular weight and for the low molecular weight component. This polymer comprises a recurring structural unit of formula (I) 
wherein
R1 and R2, independently of each other, denote hydrogen or a C1-C6 alkyl, preferably a C1-C4 alkyl,
R3 and R4, independently of each other, represent hydrogen or a C1-C6 alkyl, preferably a C1-C4 alkyl, particularly methyl and/or ethyl, or R3 and R4 jointly represent an alkylene, preferably a C3 or C4 alkylene (comprising a condensed-on 5- or 6-membered cycloaliphatic ring),
R5 represents hydrogen or a C1-C6 alkyl, preferably a C1-C4 alkyl,
R1, R2 and R3, independently of each other, represent hydrogen or methyl in particular.
Apart from stereoregular head-to-tail linkages, the concatenation of the above structural units can comprise a small proportion of head-to-head linkages. The vinylcyclohexane-based polymer can be branched via centres, and can have a star configuration structure for example.
Comonomers can be contained in an amount which generally ranges from 0 to 80% by weight, preferably from 0 to 60% by weight, most preferably from 0 to 40% by weight, with respect to the finished polymer. Polymers are preferred which comprise recurring structural units of formula (I) and which are formed from one monomer or from a mixture of monomers.
The following substances can preferably be used as comonomers and incorporated in the polymer during the polymerisation of the starting polymer (a polystyrene which is optionally substituted): olefines which generally comprise 2 to 10 C atoms, such as ethylene, propylene, isoprene, isobutylene or butadiene for example, C1-C8, preferably C1-C4 alkyl esters of acrylic or methacrylic acid, unsaturated cycloaliphatic hydrocarbons, e.g. cyclopentadiene, cyclohexene, cyclohexadiene, norbomene which is optionally substituted, dicyclopentadiene, dihydrocyclopentadiene, tetracyclodecene which is optionally substituted, styrenes with alkylated nuclei, xcex1-methylstyrene, divinylbenzene, vinyl esters, vinylic acids, vinyl ethers, vinyl acetate, vinyl cyanides such as acrylonitrile or methacrylonitrile, maleic anhydride, and mixtures of these monomers.
The polymers can have a linear chain structure, or can also comprise branching sites due to co-units (e.g. graft copolymers). The branching centres may comprise star configuration polymers or branched polymers. The polymers according to the invention can comprise other forms of what is a primary, secondary, tertiary or optionally a quaternary polymer structure, such as a helix, a double helix, a folded sheet, etc., or mixtures of these structures.
Homopolymers formed from a monomer corresponding to formula (I) are particularly preferred.
Polymers which are particularly preferred for the high molecular weight component include homopolymers of a monomer corresponding to formula (I), most preferably hydrogenated styrene-isoprene polymers, particularly block copolymers, and can be used either on their own or as a mixture.
Polymers which are particularly preferred for the low molecular weight component include homopolymers of a monomer corresponding to formula (I), (block) copolymers, most preferably hydrogenated styrene-isoprene polymers, particularly hydrogenated styrene-isoprene polymers which comprise 3 to 8, preferably 3 to 5, radial members. The low molecular weight component can be present either as one polymer or as mixtures of said polymers.
The vinylcyclohexane-based polymers can have an atactic, a predominantly syndiotactic or a predominantly isotactic diad configuration.
Amorphous substrates are also preferred which comprise a predominantly syndiotactic configuration of the vinylcyclohexane units and which are characterised in that the amount of diads is greater than 50.1% and less than 74%, most preferably greater than 52% and less than 70%.
Methods of elucidating the microstructure by means of 13Cxe2x80x941H correlation spectroscopy of the methylene carbon atoms of a polymer backbone are generally known and are described by A. M. P. Ros and O. Sudmeijer (A. M. P. Ros, O. Sudmeijer, Int. J. Polym. Anal. Charakt. (1997), 4, 39) for example.
The signals of crystalline isotactic and syndiotactic polyvinylcyclohexane are determined by means of two-dimensional NMR spectrometry. In the 2D CH correlation spectrum, the methylene carbon atom (in the polymer backbone) of isotactic polyvinylcyclohexane splits into two separate proton signals, and indicates a purely isotactic diad configuration. Syndiotactic polyvinylcyclohexane, on the other hand, only exhibits one signal for the C 1 carbon atom in the 2D CH correlation spectrum. The amorphous syndiotactic-rich polyvinylcyclohexane according to the invention exhibits an integral excess of intensity of syndiotactic diads compared with the isotactic diad configuration.
VCH polymers are produced by polymerising derivatives of styrene with corresponding monomers, by a radical, anionic or cationic mechanism, or by means of metal complex initiators or catalysts, and by subsequently completely or partially hydrogenating the unsaturated aromatic bonds (see, for example, WO 94/21694, EP-A 322 731).
Polymers based on vinylcyclohexane are produced in particular by the hydrogenation of styrene derivatives which have been polymerised by an anionic or radical mechanism. The polymers according to the invention can comprise bimodal or optionally multimodal distributions over the range of polydispersity considered.
One skilled in the art in the field of anionic and radical polymerisation is aware that the polydispersities of prepolymers can be adjusted between 1 and 3 (Braun, D., Praktikum der makromolekularen organischen Chemie, revised, expanded edition, Heidelberg, Huethig 1979).
The absolute (weight average) molecular weights Mw of the hydrogenated products are determined by light scattering. The absolute (number average) molecular weights Mn are determined by membrane osmosis (vapour pressure osmosis). Another method of characterising the molecular weight distribution by polydispersity is the measurement of the relative molecular weights Mw and Mn by gel permeation chromatography.
The process generally results in practically complete hydrogenation of the aromatic units. As a rule, the degree of hydrogenation is xe2x89xa780%, preferably xe2x89xa790%, most preferably  greater than 99% to 100%. The degree of hydrogenation can be determined by NMR or UV spectrometry, for example.
The melt viscosity is determined by the oscillation method, by means of a melt viscometer of plate and cone construction. The viscosity depends on the shear rate (angular frequency), and the viscosity at 300xc2x0 C. and 1000 secxe2x88x921 of the polymers or mixtures according to the invention is generally up to 1000 Paxc2x7s, and preferably ranges from 5 to 500 Paxc2x7s, most preferably from 10 to 200 Paxc2x7s.
The starting polymers are generally known (e.g. WO 94/21 694).
Polymerisation can be conducted continuously, semi-continuously or batch-wise.
The amount of catalyst used for hydrogenation depends on the process employed; the latter can be carried out continuously, semi-continuously or batch-wise.
In a batch process, the ratio of catalyst to polymer is generally 0.3-0001, preferably 0.2-0.005, most preferably 0.15-0.01.
The polymer concentrations with respect to the total weight of solvent and polymer generally range from 80 to 1, preferably 50 to 10, particularly 40 to 15% by weight.
The starting polymer is hydrogenated by methods which are generally known (e.g. WO 94/21 694, WO 96/34 895, EP-A-322 731). A multiplicity of known hydrogenation catalysts can be used as catalysts. Examples of preferred metal catalysts are cited in WO 94/21 694 or WO 96/34 896. Any catalyst which is known for hydrogenation reactions can be used. Suitable catalysts include those with a large surface area (e.g. 100-600 m2/g) and a small average pore diameter (e.g. 20-500 xc3x85). Other suitable catalysts include those with a small surface area (e.g. xe2x89xa710 m2/g) and large average pore diameters which are characterised in that 98% of the pore volume comprises pores with pore diameters larger than 600 xc3x85 (e.g. about 1000-4000 xc3x85) (see, for example, U.S. Pat. No. 5,654,253, U.S. Pat. No. 5,612,422, JP-A 03076706). Raney nickel, nickel on silica or on silica/alumina, nickel on carbon as a support, and/or noble metal catalysts e.g. Pt, Ru, Rh, Pd, are used in particular.
The reaction is generally conducted at temperatures between 0 and 500xc2x0 C., preferably between 20 and 250xc2x0 C., particularly between 60 and 200xc2x0 C.
The solvents which are customarily used for hydrogenation reactions are described in DE-AS 1 131 885 for example (see above).
The reaction is generally conducted at pressures from 1 bar to 1000 bar, preferably from 20 to 300 bar, particularly from 40 to 200 bar.
The vinylcyclohexane-based polymers or copolymers according to the invention are of excellent suitability for the production of optical data storage media, preferably those with densities of data storage  greater than 5 Gbyte, particularly  greater than 10 Gbyte, with respect to a disc of 120 mm diameter.
Additives, such as stabilisers and demoulding agents for example, can be added to the vinylcyclohexane-based polymers or (block) copolymers.
Examples of optical data storage media include:
magneto-optical disc (MO disc)
mini-disc (MD)
ASMO (MO-7) (xe2x80x9cadvanced storage magneto-opticxe2x80x9d)
DVR (12 Gbyte disc)
MAMMOS (xe2x80x9cmagnetic amplifying magneto-optical systemxe2x80x9d)
SIL and MSR (xe2x80x9csolid immersion lensxe2x80x9d and xe2x80x9cmagnetic super-resolutionxe2x80x9d)
CD-ROM (read only memory)
CD, CD-R (recordable), CD-RW (rewritable), CD-I (interactive), photo-CD super audio CD
DVD, DVD-R (recordable), DVD-RAM (random access memory);
DVD=digital versatile disc
DVD-RW (rewritable)
PC+RW (phase change and rewritable)
MMVF (multimedia video file system).
Moreover, due to their outstanding optical properties the polymers according to the invention are particularly suitable for the production of optical materials. e.g. for lenses, prisms, mirrors, colour filters, etc., and are also suitable as media for holographic images (e.g. for cheque cards, credit cards, passes, and for three-dimensional holographic images). The materials can be used as transparent media on which three-dimensional structures can be inscribed, e.g. three-dimensional structures from focused coherent radiation (LASER), and can be used in particular as three-dimensional data storage media or for the three-dimensional imaging of objects.